ESR1 and cancer: Through studying ALA-PpIX fluorescence, PpIX intracellular localization and cell response to ALA-PDT in a panel of human breast cancer cells including ER-positive, HER2-positive, TNBC cells, we found in the present study that TNBC cells had reduced ALA-PpIX fluorescence level and were resistant to ALA-PDT compared with ER- or HER2-positive cancer cells.